Process for producing single oriented silicon steel plates low in the iron loss

ABSTRACT

A PROCESS FOR PRODUCING A SINGLE ORIENTED SILICON STEEL PLATE OR PARTICULARLY A SINGLE ORIENTED SILICON STEEL PLATE HAVING A HIGH MAGNETIC INDUCTION PRODUCED BY ADDING A SMALL AMOUNT OF AL IN THE MOLTEN STEEL CHARACTERIZED BY THAT IN HEATING STEEL PLATE IT IS PAINTED WITH AN ANNEALING SEPARATING AGENT, TO WHICH BORON OR BORON COMPOUND IS ADDED IN A PRESCRIBED AMOUNT, AND THEN FINALLY ANNEALED, WHEREBY THE IRON LOSS CAN BE REMARKABLY REDUCED.

United States Patent 3,676,227 PROCESS FOR PRODUCING SINGLE ORIENTEDSILICON STEEL PLATES LOW IN THE IRON LOSS Fumio Matsumoto, Toshiya Wada,Takaaki Yamamoto, Osamu Tanaka, Kunihide Takashima, and ToshihikoTakata, Kitakyushu, Japan, assignors to Nippon Steel Corporation, Tokyo,Japan No Drawing. Filed Oct. 31, 1969, Ser. No. 873,070 Claims priority,application Japan, Nov. 1, 1968, 43/ 79,484 Int. Cl. H01f N18 US. Cl.148-111 1 Claim ABSTRACT OF THE DISCLOSURE A process for producing asingle oriented silicon steel plate or particularly a single orientedsilicon steel plate having a high magnetic induction produced by addinga small amount of Al in the molten steel characterized by that inheating steel plate it is painted with an annealing separating agent, towhich boron or boron compound is added in a prescribed amount, and thenfinally annealed, whereby the iron loss can be remarkably reduced.

BACKGROUND OF THE INVENTION Field of invention The present invention isto provide a method for reducing an iron loss of a single orientedsilicon steel plate or particularly a single oriented silicon steelplate produced by adding a small amount of Al to the molten steel. Asingle oriented silicon steel plate is a magnetic steel plate in whichthe crystals forming the steel have an orientation represented as (110)[001] in 'Millers index and which is easily magnetized in the directionof the rolling of the plate. The magnetic characteristics required of asingle oriented silicon steel plate are a high magnetic induction and alow iron loss characteristic. The iron loss is an importantcharacteristic influencing the efficiency of a transformer, generator ormotor made of such a single oriented silicon steel plate. The purpose ofthe present invention is to reduce this iron loss.

Description of the prior art orientation, needless to say, efiForts arebeing made so that the crystals may be well singly oriented. Theinfiuence of the crystal granularity on the iron loss is complicated. Asis well known, when the iron loss is analyzed, it is found that the ironloss consists of two factors, i.e. an eddy current loss and a hysteresisloss. As the crystal granularity becomes larger, the hysteresis loss isreduced. On the other hand, the eddy current loss tends to increase asthe crystal grain becomes larger. Particularly, when the crystal grainreaches a certain size, the eddy current loss increases quickly toexceed the reduction of the hysteresis loss, whereby the total iron lossincreases. Therefore, in order to obtain a single oriented silicon steelplate having a low iron loss, it is necessary to make the crystal grainproperly small.

As is well known, a single oriented silicon steel plate is made byutilizing a secondary recrystallization phenomenon. The mechanism forthe generation of the secondary recrystals is that, when a materialtreated by a combination of a cold-rolling and annealing is finallyannealed at a high temperature, grains having a specific orientation,that is, an orientation of [001] or slight deviations from thisorientation among primary recrystal grains having various crystalorientations, quickly grow, thereby encroaching on the matrix until thesteel plate is occupied substantially completely by only these grains.In this case, therefore, it is not possible to utilize conventionalmeasures usually adopted as effective to regulate the crystalgranularity such as the regulation of the coldrolling reduction rate,the annealing temperature or the addition of an impurity. Furthermore,there has been thus far reported no efiective method of adjusting thesecondary recrystal granularity. It is a known fact that for the purposeof inducing the secondary recrystallization it is necessary to add animpurity such as MnS, MnSe, AlN or VN to the steel so that a depositeddispersion phase may be formed. The role of such an impurity is toinhibit the growth of the primary recrystal grains of the matrix so thatthe matrix may be kept microgranular and may be easily encroached by thesecondary recrystal grains. Therefore, it is not effective to theinhibition of the growth of the secondary recrystal grains. Further,after the secondary recrystals have been formed, these deposits aredissociated and solid-dissolved in the steel or are removed from thesurface of the steel plate by diffusion.

Today, the above described view on the eddy current loss is well known.However, the present inventors have discovered that there is still afactor which performs an important action on the eddy current loss of asteel plate having a high magnetic induction as is disclosed, forexample, in the specifications of US. Pat. 3,287,183. This factor is aglassy film formed on the surface of the steel plate by the reaction ofthe steel plate with an annealing separating agent when the steel plateis painted with the annealing separating agent and is finally annealedat a high temperature.

Such a glassy film produced by the reaction of a steel plate with anannealing separating agent in the final annealing is utilizedextensively as an insulating film for single oriented silicon steelplates. Another effect of this glassy film is the reduction ofmagnetostriction. That is to say, due to the ditference in the thermalexpansion rate between the glassy film and steel, the glassy film givesa tension to the steel plate, whereby magnetostriction is reduced.

SUMMARY OF THE INVENTION With the above in mind, it is an object of thepresent invention to produce a single oriented silicon steel plate, orparticularly a single oriented silicon steel plate having a highmagnetic induction, which is characterized by having a low iron loss bymaking secondary recrystals of the steel plate properly small and byforming a stable glassy film uniformly on the surface of the steelplate.

Other objects will be made clear by the following description.

A concrete method for obtaining the above-mentioned object of thepresent invention is to paint a single oriented silicon steel plate,particularly a single oriented silicon steel plate having a highmagnetic induction, with an annealing separating agent, to which boronor a boron compound is added in a proper amount, when subjecting thesaid steel plate to final annealing. The amount of boron or a boroncompound to be added is 0.01 to 1.0% by weight of the annealingseparating agent. When the steel plate is painted with such an annealingseparating agent containing boron or a boron compound as above-mentionedand is finally annealed, a single oriented silicon steel plate,

particularly a single oriented silicon steel plate having a highmagnetic induction, will display a very low iron loss value.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the present invention themanner by which boron or a boron compound makes secondary recrystalgrains small has not been perfectly theoretically clarified. However, itis believed that boron will diffuse into the steel from the surfaceduring the final annealing and form a boride in the steel and that, evenduring annealing at a high temperature, this boride will not bedecomposed and will prevent the growth of secondary recrystal grains.The secondary recrystal grains are large in size and are inherentlydifiicult to grow due to the small orientation therebetween. It isbelieved to be due to this difficulty that the growth of the recrystalsis prevented even by the slightest amount of impurity.

As to the effect of the method of the present invention in making thecrystal granularity of the product fine, it is possible to obtain aproduct having a granularity smaller than any conventional productsaccording to A.S.T.M. by adding 0.2% boron. Examples thereof are shownin the following:

A.S.T.M. No. Amount of addition of boron (percent): (IX) 0.2 5

Further, the boron acts to improve and stabilize the properties of afilm in that the remaining boron, other than that which has diffusedinto the steel, participates in the formation of a glassy film. That is,by the addition of boron, the interlayer resistance of the glassy filmis approximately doubled, gas marks and the like vanish, fluctuations inthe quality of the film due to fluctuations in the properties of theannealing separating agents such as MgO are prevented, and in additionthereto a thin adhering uniform film is formed over the entire surface.Thus, it is, clear that boron acts to reduce iron loss by controllingthe granularity of the secondary recrystals and by the formation of ahigh quality glassy film as above-mentioned. At any rate, it has beenevidenced that, when a fixed amount of boron or a boron compound isadded to the annealing separating agent during finishing annealing, asingle oriented silicon sheet plate having a low iron loss can beobtained. The silicon steel material, which is the starting material ofthe present invention, may comprise a steel ingot made by solidifying byany casting method a molten steel prepared by a known steel making meanssuch as, for example, an open-hearth furnace, an electric furnace or aconverter or by any known melting means such as, for example, a highfrequency electric furnace or a vacuum melting furnace. A slab-formedsteel ingot obtained by a continuous casting method may also be used asa starting material of the present invention. The atmosphere in the caseof the casting is usually air but may be a vacuum or inert gas. Inshort, the ingot in the present invention may be prepared by any steelmaking, melting or casting method.

For the oriented silicon steel plate to which the present invention isapplied, it is desirable that the composition of the ingot contains, forexample, less than 4.0% silicon, 0.010 to 0.065% acid-soluble Al, andless than 0.085% C. The ingot which is made into a slab by bloomingafter being cast or which is cast into a slab is made into a hotrolledcoil by continuous hot-rolling. The hot-rolled coil is treated bycombined steps of cold-rolling and annealing. However, it is necessarythat the treating steps and conditions should be those for obtainingsingle oriented silicon steel plates having a high magnetic induction.

That is, the final plate thickness is made by one or more cold-rollings.In the case of a single cold-rolling, the coldrolling reduction rate ismade 8 1 to and in the case of two or more cold-rollings the finalcold-rolling reduction rate is made 81 to 95 after intermediateannealings are carried out one or more times. In either case, the plateis to be annealed at a temperature range of 1,000 to 1,200 C. for 30seconds to 10 minutes after the hot-rolling or between thecold-rollings, so that AlN may be deposited. After this annealing fordeposition, if necessary, the plate may be quenched from the temperaturerange of 750 to 950 C. down to 400 C. during 2 to 200 seconds dependingon the contents of C and Si.

The steel plate made to be of the final thickness by the finalcold-rolling should be made to have a carbon content of less than 0.005%by decarbonizing annealing. The surface of the steel plate after thedecarburization is painted with an annealing separating agent to preventthe steel plate from sticking due to burning in the finishing annealing.Boron or a boron compound, which is an additive for obtaining a low ironloss as a characteristic of the present invention, is mixed into thisseparating agent to be applied onto the surface of the steel plate. Theannealing separating agent may be a composition including any one or acombination of any of MgO, CaO, A1303 and TiO' Boron or a boron compoundis added in the form of a powder or an aqueous solution to the annealingseparating agent. With the addition of less than 0.01% boron to theannealing separating agent, no effect can be recognized. On the otherhand, with the addition of more than 1% boron, the development ofsecondary recrystals is influenced and a single oriented silicon steelplate having a high magnetic induction can no longer be obtained.Therefore, the preferable amount of boron is from 0.02. to 0.5%. If 5kg. of the annealing separating agent per ton of steel are to be used,the amount of boron required is surprisingly only 0.001 to 0.0025 byweight, and a sufiicient eifect can be obtained with this amount.

The final annealing should be carried out at temperature and for timesufiicient for 001 secondary recrystals to develop and for impurities tovanish due to the purification and annealing. For this purpose, it isnecessary to anneal the plate above 1,000 C. for more than 5 hours in anatmosphere of hydrogen or nitrogen.

The present invention has been explained primarily with reference tooriented silicon steel plate containing Al. However, the presentinvention is not limited to such plates, but rather includes all singleoriented magnetic steel plates.

EXAMPLE 1 A plate made by blooming and hot-rolling a silicon steel ingotcontaining 0.047% C., 2.80% Si, 0.030% A1 and 0.030% S was annealed at1,130 C., pickled, cold-rolled to be of a final plate thickness of 0.35mm. and then annealed to be decarburized in a hydrogen atmosphere at 850C. for 3 minutes. When this plate was painted with an annealingseparating agent (MgO), into which 0.01 to 0.7% by weight boron had beenadded in the form of a powdery ferroboron of less than 200 meshes, andwas finally annealed at 1,200 C., the results as shown below wereobtained.

It is evident from these results that, when an annealing separatingagent containing boron in the range of the present invention is used,there can be obtained a steel plate having a lower iron loss than anyconventional steel plate painted with an annealing separating agentcontaining no boron.

EXAMPLE 2 Two plates of final plate thicknesses of 0.35 and 0.23 mm.,respectively, were made by rolling a silicon steel ingot having acomposition of 0.04% C., 2.8% Si, 0.034% A1 and 0.027% S, the rest beingFe, by the same steps and conditions as shown in Example 1, and wereannealed to be decarburized. Then, they were painted with an annealingseparating agent (MgO) in the form of a slurry into which 0.2% by weightboron had been added in the form of borax, boric acid or single boron,and were finally annealed. The results as compared with the results ofthe conventional method are as follows, and it is evident that the steelplates made by the method of the present invention have lower ironlosses.

Iron loss Plate improvement thiek- Additives into Iron loss valueMagnetic mass the annealing induction (mm) separating agent WIS/50 Wn/aoWIS/50 W17/50 Bio None (conven- 0.95 1.24 19.51

tional method). 035 Borax 0.87 1.15 0.08 0.09 19.49

Boric acid.-- 0.85 1.12 0.10 0.12 19.49 Single boron 0.85 1.13 0 l0 0 1119. 47 None (conven- 0.77 1.08 19.36

tional method). 023 Borax 0.68 0.96 0.09 0.12 19.32 Boric acid 0.66 0.940.11 0.14 19. 34 Single boron 0.67 0.94 0.10 0.14 19. 33

What is claimed is:

1. In a process for producing a single oriented silicon steel platehaving high magnetic induction and low iron loss by hot-rolling a slabcontaining less than 0.085% by weight C., less than 4.0% by weight Siand 0.010 to 35 0.065% by weight acid-soluble aluminum, subjecting thethus hot-rolled steel plate to a single cold-rolling with a reductionrate of 81 to 95% or to two or more coldrollings with intermediateannealings between them, in which the final cold-rolling rate is made 81to 95% to make the final thickness of the steel plate, and in eithercase, annealing the plate at a temperature range of from 1000 to 1200 C.for from 30 seconds to 10 minutes after the hot-rolling, or between thecold-rollings, so that AlN may be precipitated in the steel plate priorto the final cold-rolling, then subjecting the cold-rolled steel platehaving the final gauge to a decarburization annealing to reduce thecarbon content of the steel to the value below 0.005% by weight, andthereupon to a final annealing at a temperature above 1000 C., theimprovement comprising the steps of:

(a) coating the surface of said steel plate with an annealing separatingagent consisting essentially of at least (1) one compound selected fromthe group consisting of MgO, CaO, A1 0 and TiO and (2) from .01 to 1.0%by weight, based upon the weight of said annealing separating agent, ofa material selected from the group consisting of boron and a boroncompound; (b) drying the thus coated steel plate prior to a finalannealing; and (c) then subjecting said steel plate to a finalannealing.

References Cited UNITED STATES PATENTS 3,105,781 10/1963 Walter 148-113X 3,114,661 12/1963 Balm 148-122 3,157,538 11/1964 Imai et al. 148-110 X3,159,511 12/1964 Taguchi et a1 148-113 X 3,207,636 9/1965 Wada et al.148-113 X 3,287,183 11/1966 Taguchi et al. 148-111 3,333,993 8/1967Kohler 148-113 3,477,881 11/1969 Wada et al 148-113 X 3,583,887 6/1971Steger et al. 148-3155 X FOREIGN PATENTS 407,663 4/1965 Japan 148-110 L.DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US.Cl. X.R.

